The invention relates to communication systems and in particular, to a cellular mobile communications system having integrated satellite and ground nodes.
The cellular communications industry has grown at a fast pace in the United States and even faster in some other countries. It has become an important service of substantial utility and because of the growth rate, saturation of the existing service is of concern. High density regions having high use rates, such as Los Angeles, New York and Chicago are of most immediate concern. Contributing to this concern is the congestion of the electromagnetic frequency spectrum which is becoming increasingly severe as the communication needs of society expand. This congestion is caused not only by cellular communications systems but also by other communications systems. However, in the cellular communications industry alone, it is estimated that the number of mobile subscribers will increase on a world-wide level by an order of magnitude within the next ten years. The radio frequency spectrum is limited and in view of this increasing demand for its use, means to more efficiently use it are continually being explored.
Existing cellular radio is primarily aimed at providing mobile telephone service to automotive users in developed metropolitan areas. For remote area users, airborne users, and marine users, AIRFONE and INMARSAT services exist but coverage is incomplete and service is relatively expensive. Mobile radio satellite systems in an advanced planning stage will probably provide improved direct-broadcast voice channels to mobile subscribers in remote areas but still at significantly higher cost in comparison to existing ground cellular service. The ground cellular and planned satellite technologies complement one another in geographical coverage in that the ground cellular communications service provides voice telephone service in relatively developed urban and suburban areas but not in sparsely populated areas, while the planned earth orbiting satellites will serve the sparsely populated areas. Although the two technologies use the same general area of the RF spectrum, they are basically separate and incompatible by design as they presently exist. At present, if a user needs both forms of mobile communications coverage, he must invest in two relatively expensive subscriber units, one for each system.
The demand for mobile telephone service is steadily expanding and with the expansion of the service, the problem of serving an increased number of subscribers who are travelling from one region to another has become of primary importance. Cellular communications systems divide the service areas into geographical cells, each served by a base station or node typically located at its center. The central node transmits sufficient power to cover its cell area with adequate field strength. If a mobile user moves to a new cell, the radio link is switched to the new node provided there is an available channel. However, if the mobile user travels into a region where all channels are busy, or that is not served by any cellular service, or, in some cases, into an area served by a different licensee/provider, then his call may be abruptly terminated.
Present land mobile communication systems typically use a frequency modulation (FM) approach and because of the limited interference rejection capabilities of FM modulation, each radio channel may be used only once over a wide geographical area encompassing many cells. This means that each cell can use only a small fraction of the total allocated radio frequency band, resulting in an inefficient use of the available spectrum. In some cases, the quality of speech is poor because of the phenomena affecting FM transmission known as fading and "dead spots." The subjective effect of fading is repeated submersion of the voice signal in background noise frequently many times per second if the mobile unit is in motion. The problem is exacerbated by interference from co-channel users in distant cells and resultant crosstalk due to the limited interference rejection capability of FM. Additionally, communications privacy is relatively poor; the FM signal may be heard by others who are receiving that frequency.
In the case where one band of frequencies is preferable over others and that one band alone is to be used for mobile communications, efficient communications systems are necessary to assure that the number of users desiring to use the band can be accommodated. For example, there is presently widespread agreement on the choice of L-band as the technically preferred frequency band for the satellite-to-mobile link in mobile communications systems. In the case where this single band is chosen to contain all mobile communications users, improvements in spectral utilization in the area of interference protection and in the ability to function without imposing intolerable interference on other services will be of paramount importance in the considerations of optimal use of the scarce spectrum.
The spread spectrum communications technique is a technology that has found widespread use in military applications which must meet requirements for security, minimized likelihood of signal detection, and minimum susceptibility to external interference or jamming. In a spread spectrum system, the data modulated carrier signal is further modulated by a relatively wide-band, pseudo-random "spreading" signal so that the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. Commonly the "spreading" signal is generated by a pseudo-random deterministic digital logic algorithm which is duplicated at the receiver.
By further modulating the received signal by the same spreading waveform, the received signal is remapped into the original information bandwidth to reproduce the desired signal. Because a receiver is responsive only to a signal that was spread using the same unique spreading code, a uniquely addressable channel is possible. Also, the power spectral density is low and without the unique spreading code, the signal is very difficult to detect, much less decode, so privacy is enhanced and interference with the signals of other services is reduced. The spread spectrum signal has strong immunity to multipath fading, interference from other users of the same system, and interference from other systems.
In a satellite communications system, downlink power is an important consideration. Satellite power is severely limited; therefore, the number of users of the satellite that can be accommodated, and consequently the economic viability of such a system, is in inverse proportion to how much satellite transmitter power must be allocated to each user. Many of the proposed mobile communications satellite systems have relied upon user antenna directivity to provide additional effective power gain. This has resulted in significant user equipment expense and the operational inconvenience of having to perform some steering or selection of the antenna to point at the satellite. Additionally, hand held transceivers are impractical because of the relatively large directive antennas required.
Thus it would be desirable to provide a cellular communications system which integrates satellite nodes with surface nodes to provide coverage of greater surface areas without requiring the use of two different systems with attendant expense and hardware requirements. Additionally, it would be desirable to provide a cellular communications system using a spread spectrum technique which can make more efficient use of existing frequency spectrum resources and result in increased privacy in communications. Additionally, it would be desirable to permit the use of a relatively low power, compact and mobile user handset having a small, non-directional antenna, one which can communicate with both the land-based stations and the satellite-based stations.